1. Field of the Invention
This invention relates to the fabrication of contact insertion strips, printed circuit cards and the like and, more particularly, to the protection of particular contact elements thereof while spraying a conformal coating material during finishing such components.
2. Description of the Related Art
One of the final steps in the fabrication of electronic circuit boards is the spraying of a conformal protective coating of polyurethane, acrylic or like material over the printed circuit conductors and components installed on the board. It is important, however, that the circuit conductors on the card edges and insertion strips be kept free from coating, since the coating introduces an insulation material which interferes with the electrical connections which must be made when the board is inserted into a connector slot or other type of electrical connector.
Various efforts and implementations have been resorted to in the past to avoid the deleterious effects of the sprayed coating introduced on the edge conductors. Masking tape has been used on both contact insertion strips and card edges. A major drawback of masking tape is operator dependence. If the tape is not properly tooled down around irregularities on the board's surface, it is highly susceptible to seepage of the coating material. When the tape is removed, it leaves adhesive residue that must be cleaned off. In general, the use of tape for masking does not allow for the consistent location of the masking.
Dip sealing has been used, primarily on contact insertion strips. This involves a hot glue melt into which the board edge is dipped. This method results in very unreliable masking dimensions. Furthermore, the glue coating is difficult to remove from the boards. Dip seal residue on contact insertion strips results in board failure. Furthermore, dip sealing requires the use of a hot pot and a venting hood, and much more handling of the circuit boards.
Reusable masks have also been employed for final sealing. These masks are effective on card edges when acrylic coating is used and there are no irregularities on the board surface where the mask touches, such as conductor traces. Acrylic coating material exhibits less of a capillary effect than polyurethane material. However, these masks are not effective when a polyurethane coating is used. Furthermore, they are ineffective with either coating type on contact insertion strips. Moreover, a given size of mask is usable on only one board thickness.
Thus, the various approaches known in the prior art for solving the problem of protecting circuit board edge connectors against the effects of spraying the board with a protective conformal coating have not been satisfactory. What is needed, therefore, is an effective way of protecting the edge connectors during the spraying step which is reliable, repeatable and economical in terms of material and labor required. If a prefabricated mask is used, it should be adaptable in size to different board thicknesses.